The increasing concentration of population in urban settings over the past several decades has presented important environmental problems. Prominent among such problems is one presented when it becomes desirable, from a public health or aesthetic consideration, to remove unwanted odorous constituents from a gas stream, prior to its release into the atmosphere.
Various techniques have been developed in response to the need for odorous constituent removal. As a general rule, an effective technique should be tailored to the particular constituent to be removed.
For example, in the case of odors carried by air which has been in contact with untreated sewage in pumping stations, several considerations are in order. To assess each properly, it is important to identify and characterize the odor causing constituent before attempting to develop techniques for its removal. Thus, in the sewage system environment, a primary cause of odor is hydrogen sulfide. This compound is detectable by the human olfactory sense at very low concentrations. In addition to the unpleasant odor associated with it, hydrogen sulfide is noted for its toxicity and its capacity for corroding materials with which it comes in contact.
Of course, hydrogen sulfide is not the only undesirable constituent found in effluent gases. Others, such as amines, mercaptons and organic acids can be produced from a variety of sources, including rendering plants, kraft pulp plants, paint and coating operations and oil refineries. Because of the undesirability of introducing such undesirable constituents into the atmosphere, communities and governmental agencies have formulated criteria for their regulation. A suitable odor control system, meeting such criteria, would substantially reduce the likelihood of any public nuisance or annoyance by removing, in a cost effective manner, substantial amounts of unwanted substances, preferably at or near the source of their production.
Removal of odorous constituents from a waste gas stream can be accomplished by several techniques. In general, conventional odor control systems can be divided into five major classes as follows:
A. Incineration. In this technique, thermal oxidation occurs when the temperature of the gas stream is elevated to a level at which compounds burn in the presence of oxygen derived from the atmosphere or from the stream containing the odorous constituents. High temperatures are required. Although this technique is effective in controlling odor to almost any desirable level, fuel costs are generally high, even when heat recovery methods are employed.
Thus, although incineration is suitable for some applications, it is not generally the technique of choice in many odor control applications.
B. Adsorption. This comprises a process in which gaseous constituents are trapped and oxidized on the surface of a solid. Commonly used sorbents are activated charcoal and carbon. In general, systems utilizing adsorption are easy to operate and sometimes desirable because no liquid waste is produced. However, because frequent regeneration of the sorbent is required, adsorptive systems are not generally desirable as a primary odor control system.
C. Dilution. In this technique, odorous gas is mixed with enough fresh air to reduce odor concentration below a threshold level. Such a technique is not acceptable, at least in some applications, because unacceptably large volumes of fresh air would be required to achieve a desired level of odor control.
D. Masking. Odor modification or masking is a method in which a pleasant odor is superimposed on an unpleasant one. The technique is based on the premise that a person perceives a mixture of smells as a single odor. Masking is impractical for odor control at stations where large volumes of odor contaminated air are being discharged.
E. Absorption. Absorption, or chemical oxidation, is a process in which odorous compounds in a gas stream are transferred into a liquid solution and are chemically oxidized in the liquid phase. In general, a liquid solution of chemical reagents is used.
In some cases, the absorption method, utilizing chemical reaction and oxidation, is the technique of choice because of cost considerations, especially when large gas volumes having relatively low concentrations of odorous compounds are involved. In general, the technique is utilized in a mass transfer system conventionally known as a scrubber.
In scrubber systems utilizing the absorption method, reaction between chemical treating agents and the odorous constituents takes place in the liquid phase. Removal efficiency depends on the transfer rate of the compounds from air into liquid. This, in turn, is dependent on mass transfer coefficient and total interfacial surface area. Such considerations drive scrubber design.
Several conventional scrubber systems exist for bringing chemical reagents into contact with an air stream bearing odorous compounds. Such systems include packed towers, spray/mist chambers, Venturis and impingement tray towers.
A conventional chemical treatment and oxidation scrubber currently in widespread use is a scrubber utilizing a packed tower with a fan for exhausting odors from a contaminated area. Often, such a scrubber comprises a large cylindrical tower having random packing irrigated by a recirculating reagent solution from a liquid sump located below the packing. The random packing, as opposed to structured packing, is used to maximize surface contact between the gas undergoing treatment and the reactant liquid. In such a system, odorous gas is passed through the unit and contacted with the recirculating liquid stream in a counter-current fashion. Odorous constituents are absorbed into the liquid where they react with chemicals added to the recirculating liquid. The treated gas exits at the top of the scrubber and by-product reactants are accumulated in the liquid in the sump until purged from the system by a blowdown stream.
In some cases, scrubbers utilizing packed towers are added as a retro-fit project in response to neighbor or employee complaints. In other cases, of course, such systems are contemplated in the initial design of a facility and are erected during facility construction. In either case, a typical packed tower system presents significant height and floor space requirements. In other words, such systems often have a "footprint" which is substantially greater than one might wish.
For example, a typical 15,000 cfm packed tower system for a municipal wastewater treatment plant requires more than 20 feet of height clearance and, with ancillary equipment, occupies approximately 400 square feet of floor space. Thus, it is apparent that such conventional systems, although having substantial utility, impose a large sacrifice of expensive facility space and volume. The problem is compounded when a plurality of packed towers is required, either because of the nature of the unwanted substances in the gas being treated or because reaction products formed within one packed tower must, in turn, be treated or neutralized in another tower before release to atmosphere.
In view of the aforementioned limitations of conventional odor control systems, it would be desirable to have a system for effective treatment of odorous gases which would eliminate the necessity of dedicating large portions of a facility to the system.
In addition to the expensive and undesirable space penalties presented by conventional systems, other problems are presented by the necessity of purchasing and installing ancillary equipment. Typical scrubbers utilize one or more packed towers, each of which requires, for its operation, recirculating pumps, exhaust fans, chemical metering pumps, liquid monitors and chemical storage. These components are generally assembled at the site of erection of the scrubbing system and must by wired or plumbed into the system. In the usual case, they substantially increase the size of the footprint of the scrubber and their cost can sometimes equal or exceed the capital cost of the equipment, especially when installation costs are considered.
In cold climates, it is sometimes necessary to insulate or provide suitable enclosures for the ancillary equipment, thereby again increasing installation and operating costs. In view of these considerations, it would be very desirable to have an odor control system which would reduce substantially the costs of installing and operating ancillary equipment. Ideally, such a system would have operating efficiencies at least equal to those of conventional systems while substantially reducing installation costs and the size of the system footprint.
Another significant problem with conventional packed towers is plugging of the packing as a result of accumulation of solids. Such accumulations can quickly lead to reduced system efficiency. Plugging may be caused by a variety of operational parameters such as hardness of make-up water, the chemical reagents utilized, and system pH. It is generally recognized that plugging can be effectively reduced or eliminated by purging the system. In the purging process, an amount of the recirculation stream, together with an equal or greater amount of the by-product salts created and added to the system, must be constantly removed from the sump to prevent the accumulation of solids and resultant plugging. Fresh make-up water and new chemical reagent must be added to replace the purge stream.
However, a major concern in consideration of system purge rate is the cost of unreacted chemical reagent which is sent to the drain during the purging process. Over time, this cost can make system operation prohibitively expensive. The alternative, an inefficient, plugged system, is also unacceptable. The dichotomy presented by the desire to have a smoothly operating system on the one hand and conserving chemical reagent on the other hand, can cause friction among system managers where one may desire high purge rates (and reduced maintenance costs) while another would opt for reducing chemical reagent costs by utilizing low purge rates.
In view of the foregoing, it would indeed be desirable to have an odor control system in which packed tower purging could be accomplished with a significant reduction in reagent waste. Such a system would result in a significant reduction in the large footprint of conventional systems, would reduce or eliminate some of the problems presented by installation and operation of ancillary equipment, and would provide a method of packed tower purging that would permit suitable purge rates while reducing the amount of wasted reagent. Such a desirable odor control system would substantially lower operating costs by maximizing reagent utilization.